how to connect to port of computer which behind router

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http://www.linuxquestions.org – Greetings to all Friends people with more knowledge than me ( and sense) can help to troubleshoot it. When one should try to connect to port 3306 of particular ip here e.g. 345.56.67.87 it should be redirected to port 3306 of internal machine. This is the scene : How can I access the particular port of the machine which is behind the router . i.e. From out side internet I would like to connect to the port 21 of the machine ( which is behind router (345.56.67.87) . Here are the details I tried : both side linux (centos) 1. enabled ip forwarding of router (345.56.67.87) 2. enabl (HowTos)